Eddy current inspection is a commonly used technique for detecting discontinuities or flaws in the surface of a gas turbine engine component. Eddy current techniques are based on the principle of electromagnetic induction in which eddy currents are induced within the material under inspection. Eddy currents are induced in a test specimen by alternating magnetic fields created in the coil of an eddy current probe when the probe is moved into proximity with the component under test. Changes in the flow of eddy currents are caused by the presence of a discontinuity or a crack at or near the surface of the specimen under test. The altered eddy currents produce a secondary field which is received by the eddy current probe coil or by a sensor coil in the eddy current probe which converts the altered secondary magnetic field to an electrical signal which may be recorded on a strip chart. An eddy current machine operator may then detect and size flaws by monitoring and reading the signals recorded on the strip chart. Flaws or defects are detected if the electrical signal exceeds a predetermined voltage threshold.
Present eddy current inspection methods work satisfactorily when the components under inspection have simple geometrical shapes, such as holes, flat plates or the like. However, when the component under test has a complex geometrical shape, such as the dovetail slots of a high pressure or low pressure turbine disk, fan disk, high pressure compressor disk, teeth of a gear or the like, the complex geometry of these components such as edges and transitions between convex, concave and flat regions, produces contributions to the eddy current signals which make it difficult to distinguish between defects and geometric effects.
Such complex geometric features can produce larger eddy current signals than the signals from a specific crack or flaw of interest. This makes it difficult to distinguish geometric edge signals, for example, from a crack or seam, especially when using a signal amplitude basis for accepting or rejecting parts.
One attempt at solving this problem is described in U.S. Pat. No. 5,345,514, which builds real time images from identical, repeated geometries in a part. After the collection of several of these images, a subtraction process is initiated that subtracts the images from adjacent features. While this reduces the dominant edge signals that are common to adjacent features, it does not eliminate them due to geometric variation from feature to feature within a single part. If only one feature is to be inspected, the method does not work because there are no repeated images to subtract.
Conventional, single probe eddy current inspections have utilized numerous filtering techniques that distinguish between relevant and non-relevant signals, or filter out signals from different portions of the frequency spectrum. Yet none of these techniques have been satisfactory in reducing or eliminating edge and other geometry signals.
What is needed is a method of eddy current inspection that can recognize meaningful signals related to geometry and delete or distinguish them from spurious signals that represent cracks and other important information about the integrity of the part under test.